It is well known that magnetic fields can selectively exert forces on non-ferromagnetic substances by taking advantage of physical interactions of the magnetic field with matter. Prior arrangements have used either (1) a significant difference in magnetic susceptibility, or (2) "Eddy Current" principles. Using these forces, separators can be constructed to separate electrically conductive materials from non-conducting materials. The first of these types of separator is referred to as a gradient separator. Gradient separators are used to separate weakly magnetic substances from other substances especially where the weakly magnetic substances have large magnetic susceptibilities, X.sub.m, relative to the other substances. In the presence of a magnetic field, B, a magnetic dipole moment is induced in the magnetic substance. The magnitude of the dipole moment is proportional to the product of the susceptibility and the field, viz. X.sub.m .multidot.B. The force on the particle is a function of the product of the magnetic moment and the gradient of the magnetic field, EQU F.about.X.sub.m B.multidot..gradient.B
In recent years, high intensity, high gradient magnetic separators have been developed. Field intensities of 1-2 Tesla and gradients of 10.sup.5 Tesla/meter have been obtained in commercial applications for example, by filling the working volume with a fine, ferritic stainless steel wool. The particles are attracted to and bound to the steel wool while the magnetic field is on. The particles are flushed out of the apparatus after the magnetic field is turned off. In most cases the gradient type separator is used to remove impurities from a host material. Laboratory scale experiments on clear channel, continuous flow processes have also been reported in which very high magnetic and gradient fields have been produced by using super-conducting magnets. However, this is an extremely costly and cumbersome process which is not economical in many uses.
A second type of separator is referred to as an eddy current separator. This method and apparatus takes advantage of the fact that time varying magnetic fields induce currents in conductors and the magnetic field can exert forces on these induced currents. The time variation in the magnetic field can be caused by an explicit change in the magnitude of the field or by relative motion between the conductor and the field.
Several types of eddy current devices are known in the art. Representative of a device that depends on the explicit time variation of the magnetic field is a coil through which a capacitor is discharged. The resulting time varying magnetic field accelerates a conductor away from the coil. Some of the disadvantages of this method are that the induced currents ultimately approach zero. As a result, the size of the conductor has to be relatively large (e.g. millimeters). Also, it is relatively difficult to repetitively accelerate the conducting particle whereby repeatable operations are not easily achieved. Perhaps most troublesome, attempts to take advantage of the relative motion between the conductor and the magnetic field lead to prohibitively large relative velocities especially when applied to conductors on the order of tens of microns in size.